This invention relates to high speed optical scanners and printers. More particularly, this invention relates to high speed optical scanners and printers for use with images at near distances.
There are numerous applications for high speed optical scanners and printers including non-impact printing, color imaging, digitizing, phototypesetting, bar code reading, inspection, microscopy, photolithography, PC board generation, halftone and color separation. The manipulation and processing of the digitized data acquired from scanning and stored in mass memory units, and its subsequent printing onto film, xerographic and other media offer numerous opportunities for commercial, industrial and military products.
The various methods utilized for high speed printing and scanning include the rotating polygon, the acousto-optic deflector, the galvanometer, and the holographic deflector. More recently, scanning has also been performed using linear arrays of detectors in a charge coupled device (CCD). For printing, the rotating polygon is the standard against which most deflectors are compared.
Speed and quality of scanned and printed images are two of the scanning industry's basic performance measures. They are interrelated as high image definition is dependent upon small scanning spot size, which yields more scan lines per page and requires more time to scan. Image quality in printed media, however, is affected by additional factors, including the straightness, position repeatability and uniformity of width and spacing. Errors due to these factors are readily detectable to the human eye and, therefore, require high control precision, typically ten percent of the recording dot size (e.g., 1 micrometer for a 10 micrometer dot).
A more subtle defect in image quality is image distortion. This is extremely important in systems requiring high positional accuracy of points within the image. Image distortion arises when the length of the line segment scanned during a fixed period of time differs across the scanned line. This defect is known by various names including F-theta deviation, addressability error, non-linearity, and scan velocity error.
Image spot size is primarily determined by the magnitude of the residual optical aberrations of the system, by the relative aperture size of the image forming optics, and by the wavelength of the light employed. Short wavelengths and large relative apertures yield small spots. The relative aperture size and wavelength of light used depend on the system design. This involves format size, print medium and laser light source characteristics, system size, power, manufacturing costs and various other considerations. Accordingly, systems cannot be compared simply on a dots-per-inch ("dpi") basis.
Spot size growth beyond the theoretically achievable minimum spot size due to aberrations is, on the other hand, a valid performance measure for comparison of systems. Growth in spot size in monochromatic systems arises when the image forming light is spread from its theoretical minimum spot size by spherical aberration, coma, astigmatism and field curvature, and scan defocus.
In color systems, further spot size growth arises from chromatic aberrations. On-axis the color aberration causes symmetrical growth in spot size versus wavelength. This is due to the various wavelengths arriving at different focal planes because the lens focal length varies with wavelength. Off-axis the color aberration is particularly objectionable because the growth in spot size is asymmetrical, being linear in the direction of the scan. Again, this is because the projecting optics image size depends on the focal length which, in turn, depends on the wavelength.
The speed of scanning or printing can be limited by many system factors including sensitivity of the printing medium, intensity of the light source, and the bandwidth of the electronics utilized. Often, the speed of scanning or printing is limited by the maximum speed of the beam deflecting device.
Rotating polygon mirrors are typically used as beam deflectors. See, for example, U.S. Pat. Nos. 5,015,050 and 5,028,103. One reason rotating polygon mirrors are used is that they allow multiple lines to be scanned on each revolution, thereby increasing the line scan rate. A number of lines equal to the number of polygon facets may be scanned or printed in each revolution. The number of polygon facets, however, cannot be increased to achieve higher line rates without penalty.
The polygon facet size must be large enough to accommodate the input scan light beam diameter throughout the scan angle range. Dimensioning the polygon to accommodate the input scan light diameter increases the polygon size. Large polygons, however, are not easily driven at high speed due to the air drag caused by the large surface area. Additionally, the "paddle wheel" effect in pushing the air can create substantial drive system problems in both the bearings and drive motor. Evacuated housings are sometimes used but require special precautions and may suffer from poor reliability. Also, bearing load and residual unbalance increases with larger polygons leading to shorter bearing life.
High speed rotating polygons may also suffer from facet surface deformation, and resultant image deterioration, due to centrifugal force. Polygons may also suffer from scan line position inaccuracy due to facet angle errors. Ideally, each scanned line should exactly overlay the preceding one. Then, if the medium is moved at constant rate, uniformly spaced parallel lines and a straight vertical edge will result. Polygon facet errors, however, cause deviations in line parallelism, line spacing and start-of-line position.
Facet wedge error causes deviation in the plane of the scan and thereby affects the start-of-line position. Facet wedge error is frequently compensated for electronically with a start-of-line photodetector and appropriate synchronization circuity.
Facet pyramid error deviates the line of sight in the cross-scan direction and was a major problem in achieving good scans prior to the introduction of cylindrical and toroidal projection optics. Cylindrical and toroidal projection optics allow good line repeatability to be achieved with polygons manufactured to loose facet angle tolerances. The required lenses, however, are more complex to manufacture and yield poor color performance and non-uniform line width due to focus shifts in the tangential and sagittal focal surfaces.
In order to meet the F-theta condition, a projection lens must incorporate negative distortion in the proper amount and distribution. This imposes a considerable constraint on the lens design since it must also reduce spherical aberration, coma, astigmatism and field curvature. As a consequence, in order to achieve a high level of F-theta correction, a complex lens design involving five elements or more is required. Color correction necessitates additional lens elements and adds to the difficulty in achieving high F-theta correction while reducing all of the other aberrations. Relatively simple systems may provide 0.3% F-theta correction, but more demanding systems require less than 0.1%.
Polygon scanners are also limited to less than 60 degrees scan angle. A large scan angle is desirable because a given line length can then be scanned over a shorter distance. This results in smaller system size, always a desirable design characteristic.
For polygons, scan efficiency is approximately 50%. Scan efficiency or "duty cycle" is the ratio of the "time on" to the "time available" during which a scanner is printing or scanning. A scan efficiency of 50% means that half the light goes unused and that the data rate must be twice what it would be if the efficiency were 100%. This essentially doubles the electronic bandwidth requirement, which may become the limiting factor in achieving higher speed.
A basic compact video rate optical scanner system is described in detail in U.S. Pat. No. 4,538,181 and is shown in FIG. 11. The disclosure of this patent is incorporated herein in its entirety by reference. In this system, incoming radiation from an image is reflected off a framing mirror 20 through a meniscus lens 22, past a strip mirror 24 to an imaging reflector lens 26. The radiation is then focused on strip mirror 24 and reflected toward the concave reflectors 28 of rotating disc 30, from which it emerges as a collimated beam. The collimated beam passes the strip mirror and is focused on detector 32 by means of a collector reflector 34.
The curved strip mirror 24 is the exterior surface of a cone formed with a 45 degree half-angle and an axis which is coaxial with the axis of rotation for disc 30 as indicated by dotted lines 36. Imaging mirror 26 is a concave mirror with a spherical contour having a radius equal to the distance from the vertex of framing mirror 20. The reflected image off mirror 26 lies on a spherical surface which is intercepted by curved strip mirror 24. The radius of curvature of the image is one-half the radius of curvature of imaging mirror 26 and is equal to the scanning disc radius R.sub.D.
The scanner described in U.S. Pat. No. 4,538,181, however, suffers from significant limitations preventing its utilization in line printer and scanner applications. The scanner described in U.S. Pat. No. 4,538,181 was developed for infrared applications. Consequently, all lens materials including that of the meniscus lens, were made from material which transmits infrared radiation but not visible light, and are thus is inappropriate for optical line scanner and printer applications in the visual spectrum.
It will also be appreciated that line scanners either read or write lines at finite image distances. The scanner of the '181 patent, however, was developed to read data from a two dimensional scene format located at essentially infinite image distance. Further, the oscillating mirror which scans the vertical dimension in the image format serves no function in a line scanner.
Moreover, since the scanner optics and image are concentric about the center of the aperture stop, the image surface is also inherently concentric and spherical, whereas it is very desirable for the image surface to be flat. For distant images, such as typically found in thermal imaging, the radius of the image sphere is long enough that the depth of focus allows relatively near and far range images to be simultaneously in focus. For very near, flat images, however, such as a film plate or a line along a rotating drum, field curvature is excessive.
Accordingly, there is a need in the art for a high speed optical scanner and printer providing improved performance in image spot size, spot size uniformity, color imaging, line position repeatability, F-theta condition, line generation speed, increased scan angle, and scan efficiency.